As is known in the art, eddy current measurement techniques involve passing an alternating current through a coil which induces eddy currents in an adjacent conductive sample. The closed loop induced currents run perpendicular to the magnetic flux of the exciting coil (typically the current runs parallel to the coil windings). The conductive components of the sample modify the response of the electrical circuit and allow the resistance of the sample to be determined. The depth of the conductive material that is sensed is related to the frequency utilized to excite the coil. Two coils are often used in eddy current measurement, one coil for inducing eddy currents and another coil for sensing the change in electrical response. Such an eddy current instrument, and eddy current metrology in general, is discussed in detail in U.S. Pat. No. 4,849,694 entitled “Thickness Measurements of Thin Conductive Films” by Vincent J. Coates, issued Jul. 18, 1989, which is incorporated herein by reference. Alternatively, a single coil can be used for both inducing the eddy currents and sensing a change in the electrical response.
An eddy current sensor is commonly used to measure properties associated with conductive samples. For example, the sheet resistance associated with a thin metallic film (Rf) can be measured with an eddy current metrology tool. With a knowledge of the resistivity (ρ) of the thin film material, the resistance measurement can be converted to a thickness measurement. The following formula relates the measured sheet resistance to the thickness (x) for a thin film:
                              R          f                =                              ρ            x                    .                                    Eq        .                                  ⁢        1            Measuring the resistance and thickness of thin films is very important in the semiconductor and magnetic head industries to ensure proper process control. Eddy current measurements can also be employed to characterize the magnetic properties of samples or detect the presence of defects or voids in conductive samples.
In some applications it is desirable to use an eddy current tool to measure the resistance and/or thickness of a conductive thin film that is deposited onto a substrate that already has an appreciable conductivity. If this “background” conductivity is ignored, the eddy current measurement will incur significant error. To minimize the error associated with the conductivity of the substrate, an eddy current measurement is first made before the thin film is deposited onto the sample. The resistance associated with the substrate is called Rs. Next, the film is deposited onto the substrate. A second eddy current measurement is made at the same location measuring the total resistance associated with the thin film plus the substrate. This resistance is called Rt. The resistance associated with the thin film (Rf) is calculated using the following formula:
                              R          f                =                              1                                          1                                  R                  f                                            -                              1                                  R                  s                                                              .                                    Eq        .                                  ⁢        2            
It may also be possible to measure the resistance associated with a thin film on a conductive substrate only after deposition by making multiple measurements at different excitation frequencies. The excitation frequency (f) associated with an eddy current measurement affects the penetration depth of the eddy currents into the sample. This penetration depth is known as the skin effect. The formula for the approximate skin depth (δ) (in microns) for copper is as follows:
                    δ        =                              66,000                                f                                              Eq        .                                  ⁢        3            The higher the frequency, the shallower the eddy currents penetrate the sample. By measuring the resistance of the substrate plus thin film at different frequencies (skin depths), the resistance contribution from just the thin film can be mathematically extracted. The excitation frequencies must also be chosen so that the skin depths are optimized for the thickness of the thin film and the depth of the conductive components of the substrate. For example, a frequency of 100 MHz must be used for a skin depth of approximately 6.6 microns of copper. The obvious advantage of this technique is that a pre-deposition measurement will not need to be made improving the throughput of the metrology tool. If the eddy current tool is being used in an integrated configuration (the metrology module is integrated into a process tool) the enhanced throughput of this procedure is advantageous.
Conventionally, the eddy current metrology probe is located above a flat substrate sample to be measured. It is desirable to place the probe a fixed distance above the sample, as varying this distance will introduce errors to the measured resistance value, especially at high excitation frequencies. As discussed in U.S. Pat. No. 4,849,694, it is desirable to place the measurement probe that measures the distance from the surface, e.g., a microscope, in close proximity to the eddy current sensor to ensure that the distance of the eddy current sensor from the surface is not subject to the variation of the height of the surface over small distances.
FIG. 1 shows a conventional eddy current metrology tool 10 that includes a coil 12 mounted to a microscope objective lens 16. In general, the focusing capability of the microscope objective lens 16 is used to determine and set the eddy current sensor a fixed distance above the sample.
As shown in FIG. 1, there is a distance D (which may be 10 mm) between the measurement location of the microscope objective lens 16 and the center of the eddy current coil 12. Accordingly, if a polar coordinate type stage is used with metrology tool 10, not every location on the sample 18 will be able to be measured unless extra linear travel (10 mm in this example) is incorporated into the r axis. This extra linear travel will increase the footprint of the stage. In addition, metrology tool 10 requires that the sample 18 (or the metrology tool 10) be moved during the measurement process. After focusing the microscope objective lens 16 on the surface of the sample 18, the sample 18, or metrology instrument 10, must be moved to position the desired measurement location under the eddy current sensor. If the metrology instrument 10 (or sample 18) is not moved, the eddy current sensor may measure a location on the sample 18 that is at a different height than measured by the microscope objective lens 16, as illustrated in FIG. 1. This could be a result of topographic features on the sample or a result of tilting of the sample surface with respect to the optical axis of the metrology tool. Therefore, the sample 18 or metrology tool 10 must be moved to prevent a loss of accuracy. Accordingly, an improved eddy current metrology device that does not require sample or instrument movement with no loss of accuracy or degradation of throughput is desired.